drm/simpledrm: Synchronize access to GEM BOs

Synchronize CPU access to GEM BOs with other drivers when updating the
screen buffer. Imported buffers might otherwise contain stale data.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-6-tzimmermann@suse.de